1. Field of the Invention
The present invention generally relates to small boats comprising multiple propulsion units, such as outboard motors or stern-drives (hereinafter inclusively referred to as outboard motor), mounted at the stern.
2. Description of the Related Art
With reference initially to FIGS. 8(A) to 8(C), toe angle will be described. As shown in FIG. 8(A), two outboard motors 3a, 3b are installed on a transom plate 2 of a hull 1. The outboard motors 3a, 3b are installed in a non-parallel configuration with the distance between the rearward portions of the outboard motors being shorter than the distance between the forward portions of the outboard motors. The toe angle refers to the angle theta (θ) defined between two outboard motors 3a, 3b that have been installed in a non-parallel, symmetrically diverging configuration relative to each other. Therefore, when the boat is in the neutral position (i.e., the straight-ahead running position), the turning angle (the angle relative to the axis perpendicular to the transom plate 2) for each of the outboard motors 3a, 3b is θ/2.
FIG. 8(B) illustrates a relationship between toe angle and acceleration time. As shown, the acceleration time changes depending on the toe angle. It is believed that there is a toe angle θ1 at which the maximum acceleration performance can be attained (i.e., a toe angle at which the boat reaches a desired speed in the shortest time).
FIG. 8(C) illustrates a relationship between toe angle and top speed. As shown, the top speed changes depending on the toe angle. It is believed that there is a toe angle θ2 at which the highest top speed can be attained (i.e., a toe angle providing the highest speed).
As described above, a toe angle exists that can optimize performance, whether the performance is acceleration time or top speed. In the case of conventional small boats featuring multiple outboard motors, the toe angle of the symmetrically diverged outboard motors is adjusted on land, generally prior to the shipment from the factory, to a predetermined fixed value (generally 1 degree or less), and the fixed toe angle is maintained while the boat is under way. In other words, each of the outboard motors 3a, 3b is held at a certain angle relative to another outboard motor (in practice, the angular deviation is so small that they are almost in parallel with each other) and is steered by steering wheel operation generally without changing the toe angle in the neutral position.
FIGS. 9(A) and 9(B) illustrate changes in the orientation of the outboard motors while the boat is under way. In a boat, propeller reaction force is exerted on an outboard motor due to the rotation of the propeller, and the biasing force called “paddle-rudder effect” or “gyroscopic effect” is generated, which changes the orientation of the outboard motor to make the boat proceed while angled in certain direction. As shown in FIGS. 9(A) and 9(B), an outboard motor 3 is coupled to a steering device 15 mounted on a transom plate 2 via a steering bracket 5. The outboard motor 3 turns around a swivel shaft 6 when the steering wheel is moved to cause a steering movement.
While the boat is running straight ahead with the steering handle kept at its neutral position, the propeller reaction force (F) is exerted to apply biasing force to the outboard motor 3 and changes the orientation of the outboard motor 3. FIG. 9(A) indicates the boat running at lower speeds. In this case, the propeller reaction force (F) is small, and thus the directional displacement θ of the outboard motor 3 is small. FIG. 9(B) indicates the boat running at higher speeds or running with a heavy load. In this case, the propeller reaction force (F) is large, and thus the directional displacement θ of the outboard motor 3 is large.
In reality, an anti-vibration rubber mount is interposed between the outboard motor and the steering device. Consequently, even when the steering device is moved to attain the target turning angle that corresponds to the steering wheel operation, the actual direction of the propulsive force differs slightly from the target turning angle due to the elastic deformation of the anti-vibration rubber mount caused by the propeller reaction force. The directional deviation of the propulsive force differs depending on the speed, load, propeller configuration and water pressure.
JP-B-2959044 disclosed an electric steering device that was used on a small boat. The electric steering device uses an electric motor to cause the steering action in place of the hydraulic mechanism. Smooth operation and highly accurate controllability are obtained by using the electric steering device. Another power steering configuration is disclosed in JP-B-2739208. In this configuration, steering of the single outboard motor is assisted with an electric motor. However, the disclosed constructions do not discuss the relative angle of left and right outboard motors that are symmetrically positioned.
In the conventional symmetrical installation of multiple outboard motors to a single watercraft, the mounting angle adjustment procedure includes linking both of the outboard motors with a tie bar and altering the tie bar length to provide the appropriate relative angle between the outboard motors. However, the conventional mounting angle adjustment procedure must be performed on land after the boat operation is stopped and the hull is out of the water. In addition, once the adjustment is made and the toe angle is fixed, the boat must be operated without further modifications. In other words, the mounting angle cannot be adjusted on the water while the boat is under way.